Fuel injectors of direct injection engine systems often have piece-to-piece and time-to-time variability, due to imperfect manufacturing processes and/or injector aging, for example. This injector variability may cause cylinder torque output imbalance due to the different amount of fuel injected into each cylinder, and may also cause higher tail pipe emission and reduced fuel economy due to an inability to correctly meter the fuel to be injected into each cylinder.
To compensate for injector variability, correction coefficients that correct for injection parameters, such as injection time, may be used. For example, U.S. Pat. No. 5,176,122 discloses a method that utilizes both average correction coefficients and individual correction coefficients to correct for injector variability. To calibrate the average and individual correction coefficients, calibration injection events are carried out while the fuel supply is stopped during various conditions, such as idling. Specifically, during a calibration injection event, the fuel rail pressure is monitored as it falls from a normal operating pressure to a lower threshold as fuel injections are carried out. The fuel rail pressure drop is then used to calculate and update correction coefficients. Further, in order to identify individual correction coefficients, injection events of some injectors may be carried out, while other injectors are disabled or have their injection quantity reduced.
The inventors herein have recognized some potential issues associated with the above methods of calibrating correction coefficients. For example, during a calibration injection event, the fuel rail pressure drop is monitored from a normal operating pressure to a lower threshold pressure, since the lower threshold pressure may be limited by the inability of injectors to accurately meter fuel below a certain pressure, the amount of pressure drop available for a given calibration injection event may therefore be limited. In other words, the number and size of injections for a given calibration injection event may not be sufficient to accurately calibrate all injectors. In addition to be above issue, the individual cylinder injector calibration using injector deactivation can result in undesirable air-fuel ratio excursions, un-even torque production from cylinder to cylinder, and increased engine vibration (such as during idle conditions). Furthermore, when fuel rail pressure is maintained at normal operating pressures, a relatively small amount of fuel may be present in the fuel rail during calibration due to the relatively low fuel pressure. As such, the small amount of fuel present in the fuel rail may increase fuel metering sensitivity to engine heat, which may in turn degrade calibration results.
To address at least some of the above issues, a fuel injection system that utilizes low engine load operating conditions and a higher than normal operating fuel rail pressure for injector calibration may be used.
In one example, a method for controlling fuel injection of a direct injection fuel system, the fuel system having a fuel pump, the method comprising: variably operating the fuel pump to maintain a fuel pressure at a selected pressure; temporarily increasing pump operation to increase pressure sufficiently above said selected pressure and then reducing pump operation; during at least a fuel injection subsequent to the reduction in pump operation, correlating pressure decrease to injector operation, and adjusting fuel injection operation based on the correlation.
By using an increased pressure beyond the selected operating pressure, it is possible to have sufficiently large injection quantities to sustain sufficient fuel rail pressure drop with the subsequent injections for accurate injector calibration. This may especially be true when using a turbocharger to boost intake air pressure, in which case injection pressure must overcome the boosted air pressure in the cylinder. Additionally, it may also be possible to have sufficient number of calibration injections when a multitude injection per injector is desired for accurate calibration and/or multiple injectors need to be calibrated. Furthermore, it may also be possible to avoid or reduce operating injectors at pressures below that appropriate for the current operating conditions. In one particular example, by carrying out such operation during lower load conditions, it is possible to provide sufficient pressure increase for injector calibration, since lower operating pressures may be used at such lower load conditions, i.e., the operating pressure (before the temporary pressure increase for calibrating injectors) is low enough to provide sufficient room to achieve a desired pressure increase to carry out accurate injector calibration.
Furthermore, by using a higher starting fuel rail pressure during the correlation, the fuel rail can be filled with fresher and colder fuel while the fuel injector is calibrated. This may reduce the fuel rail temperature sensitivity, and thus increase correlation results.
Finally, in an example where a predetermined number of injections for each injector in the engine are performed subsequent and proximate to the reduction in pump operation, it may be possible to avoid any torque imbalance produced when a single injector is disabled or the injection volume of a single injector is reduced.